California Code, Revenue and Taxation Code - RTC § 41078

If before the expiration of the time prescribed in Section 41076 for serving a notice of deficiency determination, the taxpayer has consented in writing to service of the notice after that time, the notice may be served at any time prior to the expiration of the period agreed upon. The period so agreed upon may be extended by subsequent agreements in writing made before the expiration of the period previously agreed upon.